nomenon as yet not well understood. Urban life with its co housing and its system of rewarding the best-trained workers courage small-family norms. Furthermore, the sex imbalance th izes much rural-urban migration, the greater knowledge and a\ contraceptives, and a greater incentive to use them tend to fertility. However, many of the migrants to urban areas are i reproductive period, and there may be a relaxation of traditiona upon fertility. TheAbsortion of Manpower into the Urban Labor Force An inevitable consequence of rapid population growth is universal problem of absorbing natives and migrants into the la cities. Many are never really "absorbed"; they are either une underemployed. At present there is very little evidence aboul variation in such categories is to be found among countries or ; within a particular country. And, of course, the problem of abs the urban labor force can never be separated from conditions i; from whence the migrants come. It is pretty well established, however, that the structural transi the labor force is accomplished mainly by inter- rather than intra-changes: most men do not make radical changes within their instead, successive cohorts enter into higher-level occupations. Li is the fact that the migrants' success in finding good (stable) jobs related to their age at arrival in the urban area. If they come at they are able to compete quite successfully with the natives. C< special importance attaches to the age of entry of men moving in labor market. The Urban System The concept of urbanization includes the system of cities—1 interdependence among the cities and their relations to hinterlam systems of cities are described in terms of the urban hierarc distribution of cities within an area and the activities, or function with size) and the spatial arrangement of urban places in te interrelationships with each other and their respective hinterlands The distribution of urban places by size in a country is indepe level of urbanization. In some countries the urban population is c in smaller cities; in others large cities dominate. Although there that can tell us what urban size distribution is most appropriate stages of economic development, several factors may be relevant. Cities of different sizes generally perform different kinds of fu best understood by reference to community of origin and of destination. Reliable information on its magnitude is generally lacking, but it represents a significant proportion of all migrations. To an extent still not determined, return migration acts as a sorting mechanism, returning the less successful and least satisfied migrants back to their communities of origin.